The importance of the resistance and particularly the capacitive property of the systemic vascular bed, especially systemic veins, has been recognized for some time: this property is involved in the regulation of cardiac output, since it affects the filling pressure of the right heart. This proposal aims at describing the mechanism(s) by which the circulation may change the filling pressure to the right heart. I approach the problem by both experimental studies and model analysis. 1) Using known naturally occurring vasoactive hormones (epinephrine, norephinephrine, angiotensin, vasopressin and histamine) I study the direct affects of these hormones on arterial pressure, venous pressure, flow and blood volume in acute dogs. The variables from these experiments will be used to describe the parameters and their changes of a lumped three element series model of the entire systemic vascular bed. 2) In a second series of experiments in acute dogs I will study the carotid sinus baroreceptor reflex control of regional blood flow and particularly blood volume. In these series, I will separate the splanchic circulation from the rest of the systemic circulation and simultaneously study the baroreceptor reflex on both of these major circuits. Arterial and venous pressures flows and volumes and their changes caused by te reflex will be obtained for both of these major circuits. The variables from these two major parallel circuits will be used to identify the individual parameters (resistance and capacitance) in each circuit.